Download 2H 2 O(g)

Thermochemistry pt 2
Calorimetry
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ΔH can be found experimentally or calculated
from known enthalpy changes
Measure heat flow with a calorimeter
Heat capacity (J/K) – amount of heat needed to
raise the temperature by one degree
Specific Heat capacity (J/g·K) – amount of heat
needed to raise the temperature of 1g of a
substance by one degree
Specific Heat
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Specific heat can be found by measuring the
temperature change (ΔT) a known mass (m)
undergoes when it loses or gains a specific
amount of heat (q)
Specific heat (c) = q / (m x ΔT)
What is the specific heat of water if 209 J of
heat is needed to increase the temperature of
50.0g of water by 1.00K?
Why does it not matter whether we work in
Celsius or Kelvin?
Concept Practice
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How much heat is needed to warm 250g of
water from 22°C to near its boiling point, 98°C?
The specific heat of water is 4.18J/g·K. What is
the molar heat capacity of water?
Answer = 7.9x104J, 75.2J/mol·K
Concept Practice
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Large beds of rocks are used in some solarheated homes to store heat. Assuming the
specific heat of the rocks is 0.82 J/g·K, how
much heat has been absorbed if the
temperature of 50.0kg of rocks increases by
12.0°C. What temperature change occurs if the
rocks emit 450kJ of heat?
Answer = 4.9x105J, 11K=11°C
Constant Pressure Calorimetry
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Assume that
calorimeter prevents
the loss or gain of
heat
qr x n completely
transferred to the
solution
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-qr x n=qs o l n
qs o l n = mcΔT = -qr x n
Concept Practice
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When a student mixes 50mL of 1.0M HCl and
50mL of 1.0M NaOH in a coffee-cup calorimeter
the temperature of the resultant solution
increases from 21.0°C to 27.5°C. Calculate the
enthalpy change for the reaction assuming the
total volume of the solution is 100mL, that its
density is 1.0g/mL and the its specific heat
capacity is 4.18J/g·°C.
-54kJ/mol
Concept Practice
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When 50.0mL of 0.100M AgNO3 and 50.0mL of
0.100M HCl are mixed in a constant pressure
calorimeter, the temperature of the mixture increases
from 22.30°C to 23.11°C. The temperature increase is
caused by the following reaction:
AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq)
Calculate the heat of the reaction assuming the
combined solution has a mass of 100.0g and a
specific heat of 4.184.18J/g·°C.
Answer = -68kJ/mol
Constant Volume Calorimetry
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Combustion reactions
are studied using a
bomb calorimeter
To measure heat of
combustion we need
the heat capacity of
the calorimeter (Cc a l)
qr x n = -Cc a l x ΔT
Remember at
constant volume q =
ΔE
Concept Practice
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Methylhydrazine (CH6N2) is commonly used as a liquid
rocket fuel. The combustion of methylhydrazine with
oxygen produces nitrogen, carbon dioxide and liquid
water. When 4.00g of the fuel is combusted in a bomb
calorimeter, the temperature of the calorimeter
increases from 25.00°C to 39.50°C. In a separate
experiment, the heat capacity of the calorimeter is
found to be 7.794kJ/°C. What is the heat of reaction
for the combustion of a mole of the the rocket fuel in
this calorimeter?
Answer = -1.30x103 kJ/mol
Concept Practice
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A 0.5865 sample of lactic acid (HC3H5O3) is
burned in a calorimeter whose heat capacity is
4.812kJ/°C. The temperature increases from
23.10°C to 24.95°C. Calculate the heat of
combustion of a) lactic acid per gram and b) per
mole.
Hess's Law
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Hess's Law = If a reaction is carried out in a series
of steps, ΔH for the reaction will equal the sum of
the enthalpy changes for the individual steps.
Combustion of methane
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
2H2O(g) → 2H2O(l)
ΔH = -802kJ
ΔH = -88kJ
CH4(g) + 2O2(g) + 2H2O(g) → CO2(g) + 2H2O(g) + 2H2O(l)
ΔH = -890kJ
Hess's Law
Hess's Law
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The number of steps does not matter
ΔH for any process can be measured as long
as we know the ΔH for each of the individual
steps
This allows us to find ΔH for reactions that are
difficult to measure.
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The combustion of carbon to form carbon
monoxide
The conversion of graphite to diamond
Concept Practice
C(s) + O2(g) → CO2(g)
ΔH = -393.5kJ
CO(g) + ½ O2(g) → CO2(g) ΔH = -283.0kJ
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Use the above reactions calculate the enthalpy
of combustion of C to CO.
C(s) + ½ O2(g) → CO(g)
Answer = -110.5kJ
Concept Practice
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Carbon occurs in two forms, graphite and
diamond. The enthalpy of combustion of
graphite is -395.kJ/mol and that of diamond is 395.4kJ/mol. Calculate the ΔH for the
conversion of graphite to diamond.
Answer = 1.9kJ
Concept Practice
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Calculate ΔH for the reaction
2C(s) + H2(g) → C2H2(g)
using the following pieces of data:
C2H2(g) + 5/2 O2(g) → 2CO2(g) + H2O(l)
ΔH = -1299.6kJ
C(s) + O2(g) → CO2(g)
393.5kJ
ΔH = -
H2(g) + ½ O2(g) → H2O(l)
285.8kJ
ΔH = -
Answer = 226.8kJ
Enthalpy
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Various ΔH measured according to the type of
process.
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ΔHv a p = vaporization
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ΔHf u s
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ΔHf
= fusion
= formation
Enthalpy changes depend on temperature,
pressure and state of reactants and products
To compare enthalpies we define a standard
state
Standard States
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A substance is at standard state when it is in its
pure form at one atmosphere and the
temperature of interest.
Standard enthalpy (ΔH°) is when all reactants
and products are in their standard states.
Standard enthalpy of formation (ΔHf°) is the
change in enthalpy that forms one mol of the
compound from its elements.
2C(graphite) + 3H2(g) + ½O2(g) → C2H5OH(l) ΔHf°= -277.7kJ
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ΔHf° of elements in their standard state is zero.
Concept Practice
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For which of the following reactions at 25°C
would the enthalpy change represent a
standard enthalpy of formation? For those that
are not, what changes should be made in the
reaction conditions?
2Na(s) + ½O2(g) → Na2O(s)
2K(l) + Cl2(g) → 2KCl(s)
C6H12O6(s) → 6C(diamond) + 6H2(g) + 3O2(g)
Calculating
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°
ΔH
rxn
from
°
ΔHf
ΔH°rxn = Σ nΔHf° (products) - Σ nΔHf° (reactants)
Calculate the standard enthalpy change for the
combustion of 1 mol of benzene to carbon
dioxide and liquid water.
Answer = -3267kJ
Standard Enthalpies of Formation
Concept Practice
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Calculate the enthalpy change for the
combustion of 1 mol of ethanol (C2H5OH)
The standard enthalpy change for the reaction
CaCO3(s) → CaO(s) + CO2(g) is 178.1kJ.
Calculate the standard enthalpy of formation of
calcium carbonate.
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Given the following standard enthalpy of
Homework
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44, 47, 49, 52, 54, 57, 59, 61, 68, 70, 75, 108,
111,